Dr Vicky Westwood, a farm vet at Penbode Vets, Holsworthy, Devon, looks ahead to protecting this year’s young lamb crop from their most pathogenic worm challenge.
Nematodirus battus (N.battus) is a particularly nasty and highly pathogenic gut worm for lambs, with the potential to cause enormous production losses, stunted growth and deaths.
Unlike other gut worms which are passed from adult ewes to lambs, N.battus is a hidden time-bomb left on pasture by the previous year’s lamb crop for this year’s young lambs.
Farm vet Dr Vicky Westwood says: “These gut worms have a simple lifecycle – lambs pass out N.battus eggs in their dung in spring and early summer.
“Eggs contain developing larvae and are especially hardy, having a double shell which helps them survive summer heat and winter cold on pasture, waiting for the following spring.
“Generally speaking, to hatch out from the eggs, larvae require a cold snap then a few spring-like days, where the temperature creeps above 10degC for 10 days in a row.
“N.battus larvae differ from other gut worms, because at hatching, they are already developed enough to cause disease in lambs grazing on pasture.
“Larvae are ingested by lambs, travel to the small intestine, develop into adults within two to three weeks, mate and shed eggs which pass out in the lamb’s dung to complete the lifecycle and contaminate the pasture for next year.”
A well-timed hatching of N.battus larvae infects young grazing lambs, typically at six to eight weeks old, causing profuse yellow/green scour and sunken-eyed, hunched and dehydrated lambs with dirty back ends.
Larvae burrow into the small intestine, damaging the gut wall and causing severe inflammation. Level of infection varies between lambs; most will recover within a month, but deaths from N.battus infection can be as high as 10-30 per cent of the lamb crop.
Lambs which do recover develop immunity to N.battus by about six months of age, which protects them throughout life against reinfection, causing retardation of adult worms and lower infection rates.
Infection is usually mild in adult animals, with low numbers of eggs shed in dung. Older lambs and ewes can help ‘mop-up’ on paddocks contaminated by young lambs the previous year.
N.battus infection is highly seasonal. Under the right conditions, a synchronised mass hatching can strike very quickly.
Because the damage is done by immature larvae which are not producing eggs, a lot of damage can be done before a standard worm egg count can detect N.battus infection.
The timing will vary around the UK; in southern England, it is more likely in April/May; in northern England and Scotland, it may be early June.
Last year, the first recorded outbreak of N.battus was in mid-March (as reported on SCOPS) in Hereford.
An online map forecasting for N.battus, updated using climate data from weather stations around the UK, is provided by ‘SCOPS’ online.
Lambs grazing pasture used for lambs the previous year are at particular risk.